Mercury is well known to be a highly toxic compound. Exposure at appreciable levels can lead to adverse health effects for people of all ages, including harm to the brain, heart, kidneys, lungs, and immune system. Although mercury is naturally occurring, most mercury emissions result from various human activities such as burning fossil fuels and other industrial processes. For example, in the United States about 40% of the mercury introduced into the environment comes from coal-fired power plants.
In the United States and Canada, federal and state/provincial regulations have been implemented or are being considered to reduce mercury emissions, particularly from coal-fired power plants, steel mills, cement kilns, waste incinerators and boilers, industrial coal-fired boilers, and other coal-combusting facilities. For example, the United States Environmental Protection Agency (EPA) has promulgated Mercury Air Toxics Standards (MATS), which would, among other things, require coal-fired power plants to capture at least approximately 80% to 90% of their mercury emissions beginning in 2015.
The leading technology for mercury control from coal-fired power plants is activated carbon injection. Activated carbon injection involves the injection of sorbents, particularly powder activated carbon, into flue gas emitted by the boiler of a power plant. Powder activated carbon (“PAC”) is a porous carbonaceous material having a high surface area, which exposes significant amounts of beneficial chemically functional and reaction sites and which creates high adsorptive potential for many compounds, including capturing mercury from the flue gas. Activated carbon injection technology has shown the potential to control mercury emissions in most coal-fired power plants, even those plants that may achieve some mercury control through control devices designed for other pollutants, such as wet or dry scrubbers used to control sulfur dioxide and acid gases.
The capture and removal of mercury from a boiler flue gas through activated carbon injection can be characterized by three primary steps, which may occur sequentially or simultaneously: (1) contact of the injected sorbent particle, e.g. an activated carbon such as PAC, with the mercury species, which is typically present in very dilute concentrations in the flue gas (e.g., <100 parts per billion); (2) oxidation of elemental mercury (i.e., Hg0), which is relatively inert and not easily adsorbed, into an oxidized mercury species (e.g., Hg+ and Hg+2), which is more readily adsorbable and is significantly more soluble in an aqueous solubilizing medium such as water; and (3) sequestration of the oxidized mercury species by the pores of the sorbent where it is held tightly (e.g., sequestered) without being released. The flue gas streams traverse the ductwork at very high velocities, such as in excess of 25 feet/second. Therefore, once injected, the sorbent must rapidly contact, oxidize and sequester the mercury. In some instances, the sorbent only has a residence time of about 1 to 2 seconds in the flue gas.
Activated carbon injection technology may be less effective in coal combustion facilities that produce flue gas streams with relatively high concentrations of acid gases and/or acid gas precursors, such as sulfur oxides (e.g., SO2 and SO3) and nitrogen oxides (e.g., NO2 and NO3). Under conditions of high temperature, moisture, and pressure such as in a flue gas, acids such as sulfuric acid (H2SO4) and nitric acid (HNO3) can form from the precursors. It is believed that these acids may inhibit or slow the mercury capture mechanism by interfering with the reaction and adsorption sites on the sorbent that would otherwise be used to capture and bind mercury. For example, it has been observed that flue gases with concentrations of SO3 as low as 3 ppm can detrimentally affect mercury capture rates.
Acid gas precursors and/or acid gases typically come from three primary sources. The first is the coal feedstock fed to the boiler as the fuel source. Certain types of coal inherently have high concentrations of sulfur, nitrogen, chlorine, or other compounds which can form acid gases in the flue gas. For example, coals such as Illinois basin coal with high sulfur content (e.g., above about 2 wt. %) are becoming more common as a boiler feedstock for economic reasons, as high sulfur coals tend to be cheaper than low sulfur coals. A second source is the selective catalytic reduction (SCR) step for controlling emissions of NOx. An unintended consequence of this process is that SO2 in the flue gas can be oxidized to form S03. A third source is that the power plant operator may be injecting SO3 into the flue gas stream to enhance the efficiency of the particulate removal devices, e.g., to avoid opacity issues and increase the effectiveness of an electrostatic precipitator (ESP) in removing particulates from the flue gas stream. Accordingly, it may not be practical for a power plant operator with any of the foregoing (or similar) operating conditions to use conventional PAC to capture mercury and cost-effectively comply with government regulations such as EPA MATS.
Several technologies have been proposed to address these situations where the presence of acid gas precursors and/or acid gases inhibits mercury capture performance. One such technology is the separate injection of dry alkaline compounds such as trona, calcium oxide, calcium hydroxide, calcium carbonate, magnesium carbonate, magnesium hydroxide, magnesium oxide, sodium bicarbonate, and sodium carbonate into the flue gas to mitigate the acid gases. Aqueous solutions may also be injected into the flue gas stream, including sodium bisulfate, sodium sulfate, sodium carbonate, sodium bicarbonate, sodium hydroxide, or thiosulfate solutions.
Another technology involves the simultaneous injection of PAC and an acid gas agent, either as an admixture or with a composition including PAC that has been treated with the agent. The acid gas agents may include alkaline compounds such as sodium bicarbonate, sodium carbonate, ammonium carbonate, ammonium bicarbonate, potassium carbonate, potassium bicarbonate, trona, magnesium oxide, magnesium hydroxide, calcium oxide, calcium hydroxide, calcium bicarbonate and calcium carbonate. Yet another technology involves the co-injection of PAC and an acid gas agent, where the acid gas agent may include Group I (alkali metal) or Group II (alkaline earth metal) compounds, or compounds including halides and a non-metal cation such as nitrogen, e.g., ammonium halides, amine halides, and quaternary ammonium halides.
The use of a halogen in sorbent compositions is a leading technology used to oxidize mercury to a form that can be captured, such as is disclosed in U.S. Pat. No. 9,539,538 to Wong et al., which is incorporated herein by reference in its entirety. In addition, in some cases the halogen, such as bromine is added separately from the sorbent as in U.S. Pat. No. 8,309,046 to Pollack et al. In some cases the halogen is present in the sorbent in concentrations of 10 wt. % or more. However, addition of halogens such as bromine to the flue gas stream may cause corrosion of treatment units. In addition, non-protective scales can be formed because of the presence of hydrobromic acid (HBr) in the flue gas. Hydrobromic acid is formed from the interaction of Br2 with water through multicomponent condensation when the flue gas temperature is below a corresponding hydrobromic acid dew point; subsequently, dew point corrosion occurs on the metal surface. At temperatures over the hydrobromic acid dew point, gaseous bromine is capable of diffusing through the oxide layer to the scale-metal interface where it reacts with the iron to form iron bromide. The volatile iron bromide may then diffuse outward to the scale surface, where it is converted to a solid oxide at the elevated oxygen concentration. The formed free bromine is either released to the bulk gas or diffuses back to the scale-metal interface, and thus a cycle is formed. The same bromine corrosion and regeneration cycles may proceed via FeBr3, and it is possible for the ferrous iron to be oxidized to the ferric state while the oxidation liberates bromine as well. In addition to the corrosive effects of adding bromine into the flue gas stream, several other plant issues can also arise; bromine accumulation in the wet scrubbers, deterioration of the fabrics in a baghouse, and the decrease of selenium capture within the plant's native fly ash.